Mosquitoes are vectors that transmit serious diseases, including West Nile virus, Zika, Dengue, and Chikungunya. Preventing adult mosquitoes begins with eliminating the aquatic stage of their life cycle, particularly the larvae. These larvae, often called “wigglers,” develop exclusively in standing water before transforming into flying adults. The aquatic phase, from egg to pupa, can take as little as five to seven days in warm conditions, making the control of standing water a time-sensitive necessity.
Routine Practices: Eliminating Small Water Sources
The most effective defense against mosquito proliferation involves regularly inspecting and emptying small, temporary water sources around the home. Female mosquitoes can lay hundreds of eggs in surprisingly small amounts of stagnant water. Common household items, such as flowerpot saucers, discarded tires, and children’s toys, are prime breeding habitats.
Any container holding water should be dumped at least once a week to disrupt the larval development period. For items like bird baths, the basin should be scrubbed vigorously before refilling to dislodge any mosquito eggs stuck above the waterline. Eggs laid by certain species can withstand drying and will hatch immediately when exposed to water again.
Routine cleaning should also extend to structural areas that trap rainwater. Clogged rain gutters prevent proper drainage, creating shallow reservoirs ideal for mosquito development. Inspecting tarps, boat covers, or plastic sheeting for pockets of standing water and ensuring they are stretched taut or drained after rainfall is necessary.
Strategies for Water That Must Remain
Controlling mosquito larvae is complex where water cannot be dumped, such as in rain barrels, swimming pools, or decorative ponds. For water collection systems like cisterns and rain barrels, physical exclusion prevents adult females from laying eggs. These containers require tight-fitting lids or screens with mesh fine enough to block entry, typically 16-mesh or smaller.
In swimming pools, consistent maintenance, circulation, and chlorination make the environment unsuitable for breeding. Poorly maintained or unused pools with stagnant water quickly become breeding grounds. Decorative water features, such as ponds or fountains, require constant water movement to prevent larval development.
Mosquito larvae are obligate air-breathers and must periodically surface to obtain oxygen through a specialized breathing tube called a siphon. Installing a fountain, bubbler, or waterfall creates enough surface disruption to prevent the larvae from reaching the air-water interface, effectively drowning them.
For larger ponds, ensuring the water is deep and free of excessive debris or vegetation helps, as larvae prefer shallow, protected areas. Another measure is addressing drainage issues by filling in low spots in the yard that collect rainwater for more than a few days.
Safe and Effective Larvicide Options
When standing water cannot be drained or maintained through physical measures, specific larvicidal agents offer a solution. The primary biological control method uses the naturally occurring soil bacterium, Bacillus thuringiensis israelensis, commonly known as Bti. Bti is sold in commercial formulations like “mosquito dunks” or “mosquito bits” and is applied directly to the water.
The bacterium produces protein crystals that are toxic only to the larvae of mosquitoes, black flies, and fungus gnats after ingestion. The alkaline environment of the larva’s gut activates the toxin, which destroys the digestive cells. This mechanism is highly specific and does not affect people, pets, fish, beneficial insects, or other wildlife, making it safe for use in bird baths and rain barrels.
For water sources not used for drinking or supporting wildlife, a thin film of mineral or cooking oil can suffocate the larvae. The oil spreads across the water surface, blocking the larvae’s siphon from drawing air, causing them to die.
Another biological option for larger, permanent ponds is the introduction of mosquito fish, such as Gambusia affinis. These natural predators consume large quantities of mosquito larvae.